As of June 17, three cases of West Nile virus (WNV) have been detected in California horses. All three cases are recovering.

The first case, a 3-year-old Quarter Horse in Plumas County, was reported on June 1. The second and third cases were from Sonoma and Kern Counties and were reported on June 10 and June 17, respectively. The Plumas County horse was vaccinated for WNV, and according to online news reports, the horse’s treating vet has credited the vaccine as increasing the horse’s likelihood for survival. The Sonoma County horse was not vaccinated against WNV. The Kern horse had been vaccinated two weeks prior to its infection, but not enough time had passed for the horse to be protected from the virus.

Twenty-seven of California’s 58 counties have found WNV in birds, horses, or mosquitoes. There are 121 positive bird cases in 25 counties, and 46 mosquito pools from eight counties have tested positive. Additionally, the virus was detected in three sentinel chickens in two counties.

California became the focus for WNV in 2004, when it logged 540 confirmed equine cases. More than 40% of clinically affected horses died or were euthanatized. West Nile virus was detected in some species in all California counties last year. The Centers for Disease Control and Prevention has predicted that California will again be the epicenter for WNV in 2005